Stumped

[kiwiNCFC]

I have been revising reactance formulas etc recently and I just went on Swindon massive to do some quick tests, and I'm not sure if im being really stupid or not but I can't get this answer:

Calculate the Power Dissipated in Resistor 2

Resistor 2 : 10 ohms

Voltage : 20v

I did P=V2/ 10 ohms which gives me 40w

But the answer it gives is 17.8w.

Hmm??

 

[matt.leung]

parallel or series ???

 

[kiwiNCFC]

series so you would add them together right? But it only asks for value of resistor 2 not resistor 1

Parallel =1/RT = 1/R + 2/R + 3/R etc

Series = ohms + ohms

 

[Andy™]

unless its 20V accross R2, you will need value of R1 to work out voltage accross R2

 

[kiwiNCFC]

Value of R1 = 5 ohms R2 = 10 ohms

 

[kiwiNCFC]

A haha there you go, now how do you work it out?

So as they are in series I can add them together ...10 + 5 = 15 ohms

P=Vsquared /R

20*20 / 15 = 26.66w

20*20 / 10 = 40w

Still none the wiser

 

[DarkInferno]

20V won't be dropped across each resistor.

 

[kiwiNCFC]

FirstTotal Resistance Rt

Second

Current through total resistance I=V/Rt

Then when you have the current ?

Total Resistance = 15 ohms

I=V/RT

I= 10v/ 15 = 0.66

P=I2 /R = 0.66x0.66 /10 = 0.04356 headbang

 

[Andy™]

Value of R1 = 5 ohms R2 = 10 ohms

so R2 will have 2/3 of the voltage... 13.3v....

so go back to your origianl calculation....

 

[Robin Spark]

[quote name='Andy

 

[kiwiNCFC]

20/15 ohms = 1.3

P= Isquared /R 1.3x1.3 = 1.69 / 10 = 0.169

hang on

13.3 x 13.3 = 176.89/10 = 17.69 W ut the kettle on

Thanks for being patient. I get it now. Just couldn't understand where the 13.3v came from? I did 20v / 2 (2 resistors) which is obviously 10 so i thought it was 10v :|

 

[Robin Spark]

1.3333 is correctYour formulae is wrong

P2= Isqrd * R2

Not div /

P2 = (1.3333 x 1.3333) * 10

He can either work out the volt drop at each resistor and then use

P= Vsquared/ r

or he can work out the current I= Supply voltage/ Rt and then

use I squared * R for that particular resistor.

Hope that makes sense???

 

[Andy™]

Thanks for being patient. I get it now. Just couldn't understand where the 13.3v came from? I did 20v / 2 (2 resistors) which is obviously 10 so i thought it was 10v :|

resistors are 10 & 5. very basic maths for this one - add together, 15. R2 is 10. so is 2/3 of the resistance. and so 2/3 of the voltage. there are other ways of working it out which may be needed for other values.

 

[Robin Spark]

Yes, more than one way to skin a cat so to speak, I wasnt thinking the volt drop route If you look at my posts and hintsThanks :Salute

As the resistors are in series then current remains equal throughout the circuit, so yes i agree your way is easier :Salute

 

[kiwiNCFC]

Thanks,

I did get it. I have not got a scientific calculator on me at the moment so i think that did not help! Using the windows basic one is awful!

Cheers guys!

 

[NozSpark]

Thanks,I did get it. I have not got a scientific calculator on me at the moment so i think that did not help! Using the windows basic one is awful!

Cheers guys!

In calculator go to

View... and select Scientific.

HTH

 

[Robin Spark]

In calculator go toView... and select Scientific.

HTH

:^O LOL I never knew that and after how many years of using windows....well its true you always learn something new each day.

thanks

 

[Andy™]

Thanks,I did get it. I have not got a scientific calculator on me at the moment so i think that did not help! Using the windows basic one is awful!

Cheers guys!

i only used a basic calc... scientific calc would require effort to get out of drawer... normal calc lives on the desk...

 

[Sidewinder]

Prefer my TI89 actually!

:coat

 

[Andy™]

Prefer my TI89 actually! :coat

bit OTT isnt it?!

whats wrong with the Abacus?!